You are going to need to provide a lot more information before anyone can help you. That's ok, when you're new, it's hard to know what is needed.

What kind of MTA are you using? (postfix, sendmail, etc.)
When did this start?
What is it being sent from? (e.g., a wordpress site, some website, just the server itself.)
It often happens when a website is hacked--if it is a website, you'll have to examine it carefully to see if you can find what was hacked.